A STUDY OF PRECURSOR CLUSTERS OF SINGLE-WALLED CARBON NANOTUBES (SWNTS) BY FT-ICR MASS SPECTROMETER

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1 The 6th ASMEJSME Thermal Engineering Joint Conference March 1620, 2003 TEDAJ03221 A STUDY OF PRECURSOR CLUSTERS OF SINGLEWALLED CARBON NANOTUBES (SWNTS) BY FTICR MASS SPECTROMETER Shuhei INOUE Department of Mechanical Engineering, The University of Tokyo, 731 Hongo, Bunkyoku, Tokyo , Japan shu18@photon.t.utokyo.ac.jp Masamichi KOHNO Research Center for Advanced Manufacturing on Nanoscale Science and Engineering, AIST Namiki 121, Tsukuba, Ibaragi , Japan masakono@aist.go.jp Shigeo MARUYAMA Department of Mechanical Engineering, The University of Tokyo, 731 Hongo, Bunkyoku, Tokyo , Japan maruyama@photon.t.utokyo.ac.jp Keywords: FTICR, Cluster, Carbon Nanotube, Fullerene, Mass Spectroscopy ABSTRACT Singlewalled carbon nanotubes (SWNTs) [1] is now recognized as the most fascinating material of socalled nanotechnology in this century. Despite the expectation of future applications, it is still difficult to obtain large amount of pure SWNTs, because the generation process is still not clear. It is well known that transition metals such as La, Y and Sc can be encapsulated inside the fullerene cage. On the other hand, transition metals such as Ni, Co or Fe are required to generate SWNTs. And the diameter distribution of SWNTs depends on the catalyst metal. However, the role of metal atom in the growth process of SWNTs is not yet known. In this paper, by using FTICR (Fourier transform ion cyclotron resonance) mass spectrometer [2], we studied laservaporized clusters from Ni/Co, Ni/Y and Rh/Pd (these metals were typical catalyst metals for SWNTs) loaded graphite samples. The FTICR mass spectrometer used in this study was based on the design in Smalley s group at Rice University [2]. Metal/carbon binary clusters were produced by laser vaporization in cluster beam source. The cluster beam was directly injected to the magnetic field through a skimmer with the opening diameter of 2 mm and a deceleration tube. After deceleration, cluster ions were trapped by electric field in the ICR cell. And cluster ions were excited for the detection of the mass distribution. Depending on metal species, generated cluster distributions were drastically different. In case of a Ni/Co doped carbon material, the peak of mass distribution appeared at relatively high mass range such as equivalent to C 150. On the other hand, for a Rh/Pd doped carbon material, the observed mass distribution of clusters were limited only up to C 70. This result can be clearly compared with the facts that SWNTs from Ni/Co doped sample has larger diameter such as 1.2 nm, in contrast to about 0.8 nm from Rh/Pd doped sample. Diameters of cage structured C 150 and C 70 are roughly 1.1 nm and 0.8 nm, respectively. Hence, it is speculated that the size of precursor clusters may determine the diameter of SWNTs. We propose that metal atoms affect the growth of clusters, and as a result, the final SWNTs diameter distribution is determined by the size of these precursor clusters. (a)pure C (b) Ni/Co/C(0.6%/0.6%) (c) Rh/Pd/C(1.0%/1.0%) Figure A1 FTICR mass spectra of as injected negative clusters from pure graphite and metaldoped graphite samples.

2 NOMENCLATURE B: Magnetic field f: Ion cyclotron resonance frequency M: Cluster ion mass m: Number of carbon atoms of a cluster n: Number of carbon atoms of a cluster q: Charge of cluster ion INTRODUCTION In 1985, when the existence of fullerene C 60 and C 70 were reported by Kroto et al. [3], this discovery of third allotropes of carbon excited a whole researchers. Later derivatives of fullerene such as heterofullerene [4], endohedralmetallo fullerene [5], and higher fullerene [6] are discovered and extensively studied. Various applications for fullerene are being explored such as a contrast medium of MRI [7,8], high temperature superconductor [9] or carriers of the medicine [10]. Afterwards, singlewalled carbon nanotubes (SWNTs) was discovered by Iijima et al. [1] and the research became more and more popular as an essential new material. Carbon nanotube is classified into two types, one is SWNTs [1] and the other is multiwalled carbon nanotubes (MWNTs) [11]. The diameter is about 0.7 nm 2.0 nm for SWNTs and 4 nm 100 nm for MWNTs, and the length reaches an order of a few tens of µm. SWNTs are expected as a new material from various physical and chemical properties based on its geometric structure. As possible applications, electric wire for integrated circuits, field effect transistor, a field emission electron source for flat displays, a probe of AFM, a heat conduction element, conductive composite material and hydrogen storage material are discussed. Despite many ideas of promising applications of SWNTs, the controlled synthesis and the bulk production at a low cost are still difficult. Even though many generation mechanisms of SWNTs (such as Kataura et al. [12] and Yudasaka et al. [13, 14]) are proposed, each idea is lacking for the decisive evidence. Especially, the role of catalytic metals such as Ni, Ni/Co, and Rh/Pd for laseroven technique and Ni/Y for arcdischarge technique is still unknown. By using a FTICR (Fourier transform ion cyclotron resonance) mass spectrometer directly connected with a laser vaporization cluster beam source, we have studied metal/carbon mixed samples, La/C, Y/C, Gd/C, Ce/C, Ca/C, and Sc/C [15, 16] for the generation machanism of endohedral metallofullerene. The metalcomposite clusters showed magic numbers of MC 44, MC 50, MC 60, (M = La, Y, Sc, Gd, Ce). The evennumbered carbon atoms for metallofullerene and the minimum size of Fast Pulsed Valve Expansion Cone Gears Waiting Room Target Disc Window Window Feedthrough for Updown Vaporization Laser To ICR Cell Gears Feedthrough for Rotation Figure 1 Laservaporization cluster beam source. 6 Tesla Superconducting Magnet Cluster Source Gate Valve Gas Addition Deceleration Tube Front Door Excite & Detection Cylinder Screen Door Rear Door Electrical Feedthrough Probe Laser Ionization Laser Turbopump 100 cm Figure 2 FTICR apparatus with direct injection cluster beam source.

3 dimetallic fullerene strongly suggested that all of these clusters had the caged form with one or two metal atoms inside. In this report, we have studied metalcarbon binary clusters generated from Ni doped, Ni/Co doped, Ni/Y doped, and Rh/Pd doped graphite, which are used for the macroscopic generations of SWNTs by laseroven or arcdischarge techniques. EXPERIMENTAL The experimental apparatus is based on the same design concept as the apparatus in Smalley s group [2] and the detailed characteristics are described in elsewhere [1520]. Figure 1 and Fig. 2 show the cluster beam source and direct injection FTICR apparatus, respectively. The carbon cluster ion beam was generated outside of magnetic field by the laservaporization cluster beam source shown in Fig. 1. A pulsed gas valve, the sample motion mechanism and a skimmer were installed in a 6inch 6way UHV cross. A solid sample disk was vaporized by the focused beam of Nd: YAG laser (2nd harmonics) while timed pulsed gas was injected to the nozzle. In the atmosphere of helium gas, vaporized atoms condensed to clusters, and then, were carried and cooled by the supersonic expansion of helium gas. The cluster beam was directly injected to the magnetic field through a skimmer with the opening diameter of 2 mm and a deceleration tube [15]. The FTICR is the unique mass spectroscopy based on the ioncyclotron motion of clusters in a strong magnetic field. The ion cyclotron frequency f is inversely proportional to the ion mass M as follows. qb f = (1) 2π M Extremely high massresolution at high massrange (a) Positive C 60 C 70 such as resolution of 1 amu at 10,000 amu range can be obtained. Furthermore, since the ions can be trapped in the vacuum for a few minutes, it is possible to perform the chemical reaction experiments. The ICR cell, 42 mm I.D. 150 mm long cylinder was placed in a stainless tube (SUS316) of 84 mm I.D. which penetrated the homogeneous Tesla superconducting magnet commercially available for NMR. Two turbopumps (300 l / s ) forepumped by a smaller turbopump of 50 l / s were placed at the floor in order to avoid the effect NiC CoC 38 NiC 39 NiC Cluster ion mass (amu) Figure 4 Expanded view of Figure3 (b) NiC 38 ~ CoC 38 ~ ~ C 36 (b) Negative (a) (b) 2s (c) 10s ~ ~ ~ NiC 38 (NO) CoC 38 (NO) ~ ~ ~ C 43 (NO) Figure 3 Positive (a) and negative (b) clusters generated from Ni/Co doped carbon materials Cluster ion mass (amu) Figure 5 NO Reaction of NiC 38 and CoC 38 from Ni/Co doped sample.

4 of strong magnetic field. The typical background pressure was Torr. For the chemical reaction experiments, the kinetic energy of clusters was thermalized with room temperature argon gas at 10 5 Torr. Then, nitric oxide (NO) gas was supplied to the cell by a pulsed valve for a fixed period. The pulse value was adjusted so that the pressure at the ICR cell chamber became at 10 5 Torr for unreactive clusters and 10 7 Torr for reactive clusters. After pumping out, cluster ions were excited to detect the mass distribution. RESULTS AND DISCUSSIONS Figure 3 shows mass spectra of clusters generated from Ni/Co (0.6 at. % Ni and 0.6 at. % Co) doped carbon material used for the laseroven SWNTs generation [21]. Positive (a) and negative (b) mass spectra as injected from the cluster beam source are shown in Fig. 3. There was not a trace of Ni or Co in the positive mass spectrum and C 60 and C 70 were observed as the magic number. This tendency was similar to the case of using pure graphite materials, though signals of C 60 and C 70 were much enhanced with this doped case. On the other hand, in the case of negative ions, tiny signals of NiC n and CoC n were measured in Fig. 4 (expanded view of Fig. 3(b)). Other drastic effect of doping of Ni/Co was observed in pure carbon cluster distribution. The evenodd alternations were observed from C 36. When the clusters were generated from the pure graphite materials, this evenodd alternation was not observed. There were some evenodd differences in the signal intensity of NiC n and CoC n ; these peaks were stronger for evennumbered carbon clusters. However, this evenodd difference seems to simply reflect the relative abundance of pure carbon clusters. The chemical reaction of these clusters with NO was used as the probe of the structure of clusters. Reaction with 10 7 Torr of NO gas for 2 s and 10 s for the case of Ni/Co doped sample are shown in Fig. 5. The chemisorption of NO to NiC n and CoC n were observed much faster rate than pure carbon clusters. After the reaction for 2 s, the product of C n NO gradually appeared. Since it is expected to be more reactive if a metal atom is exposed to NO, the high reactivity of NiC n and CoC n strongly suggests that Ni or Co atom is outside the carbon cage. The Ni/Y (4.2 % Ni and 1.0 % Y) doped carbon material optimized for the arcdischarge SWNT generation [22] was also studied. The positive mass spectrum shown in Fig. 6(a) was not distinguishable from the Y doped carbon material (0.8 % Y) used for the endohedral metallofullerene generation [16]. Most of clusters had one Y atom and even number carbon atoms, and YC 44, YC 50, YC 60 were observed as magic numbers. On the other hand, only small signals of pure carbon clusters such as C 60 and C 70 were observed. In the negative spectrum, however, small pure carbon clusters and Ni attached clusters in addition to YC 2n were observed. As shown in Fig. 7(a), abundance of Ni attached clusters were sometimes almost the half of oddnumbered pure clusters for around NiC 40, but Niattachment was not observed for large clusters more than 50 carbon atoms. Figure 7 shows the reaction of binary clusters from Ni/Y doped sample with 10 5 Torr in NO gas for 5 s. It should be noted that the pressure of NO gas was 100 times higher compared with the case of Ni/Co doped samples. Even though tiny NiC n clusters such as NiC 44 or NiC 46 were completely reacted away, the reaction of YC 2n such as YC 42 with NO was not observed. Comparing signal heights of YC 2n and C 2n, it can be concluded that YC 2n is less reactive than pure carbon (a) Positive YC 50 YC 44 C 50 YC 44 C 44 C 36 YC C (b) Negative (a) As injected C50 NiC 44 YC 42 NiC 46 C 49 (b) NO 5 s YC 42 C 50 C 51 YC 44 C 52 No Signal YC 42 (NO) C Figure 6 As injected positive (a) and negative (b) clusters from Ni/Y doped carbon material Figure 7 NO reaction of NiC n and YC n clusters from Ni/Y doped sample.

5 clusters. Hence, it is reasonable to assume that Y atom is inside the cage carbon cluster, just like the precursor clusters of endohedral metallofullerene [16]. Besides the experimental study, molecular dynamics simulations of growth process of binary clusters from randomly distributed carbon and metal atoms were performed [23]. The Y containing carbon cage cluster for YC system and carbon cage structure with Ni atom staying on a face for NiC system were predicted. These simulation results are completely consistent with experimental results. Figure 8 is an example of detected positively charged cluster ions. Laser vaporized clusters from graphite materials, (a) pure graphite, (b) Ni/Co (0.6 at. % each) doped graphite, (c) Rh/Pd (1.0 at. % each) doped graphite, (d) Ni/Co (6.0 at. % each) doped graphite and (e) Ni (1.2 at. %) doped graphite were compared. Figure 8 shows metaldoped graphite samples are much easier to generate small clusters than pure carbon. Since metalattached carbon clusters are observed in those small mass ranges, it is assumed that those metals have tendency to prohibit the further growth. This feature is completely opposite to the effect of metals used as endohedral metallofullerene such as La, Y, and Sc[16]. Figure 9 shows detected negatively charged cluster ions. Laser vaporized clusters from graphite material, (a) pure graphite, (b) Ni/Co (0.6 at. % each) doped graphite, (c) Rh/Pd (1.0 at. % each) doped graphite, (d) Ni/Co (6.0 at. % each) doped graphite and (e) Ni (1.2 at. %) doped graphite were compared. In case of Ni/Co (a) pure C (0.6 at. % each) doped carbon material, the peak of mass distribution appears at relatively high mass range such as equivalent to C 150, and Ni doped carbon material is also comparative. On the other hand, for Rh/Pd doped carbon material, observed mass distributions are limited only up to C 70. And the case of Ni/Co (6.0 at. % each) doped carbon material, observed mass distributions were further limited up to C 50. These results except for the case of Ni/Co (6.0 at. % each) are remarkably compared with the diameter distributions of macroscopic SWNTs generated with laser oven technique; peak diameter at 1.2 nm from Ni/Co doped graphite and at 0.8 nm from Rh/Pd doped graphite [12]. Diameters of cage structured C 150 and C 70 are roughly 1.1 nm and 0.8 nm, respectively. Hence, it is speculated that the size of precursor clusters may determine the diameter of SWNTs. Similar growth model has been proposed by Kataura et al. [12] that a collision of imperfect fullerene to a large metalcarbon binary particle leads to the nucleation of SWNT. Our molecular simulation [23] suggests that the imperfect fullerene clusters with a few metal atoms collide with each other to form some tubular structure. From the cluster size distributions for Ni/Co (6.0%), the thin tube might be possible, if the quantity of Ni/Co doping is increased. CONCLUSION By using FTICR, mass spectrometric analysis and chemical reaction experiment of laservaporized metal carbon mixed sample have been performed. Depending (a) pure (b) Ni/Co/C(0.6%,0.6%) (b) Ni/Co/C(0.6%/0.6%) (c) Rh/Pd/C(1.0%,1.0%) (d) Ni/Co/C(6%/6%) (c) Rh/Pd/C(1.0%/1.0%) (d)ni/co/c(6%/6%) (e)ni/c(1.2%) (e) Ni/C(1.2%) Figure 8 As injected positive carbon clusters. Figure 9 As injected negative carbon clusters.

6 on metal atoms, generated cluster distributions were drastically different. In case of a Ni/Co doped graphite material, the peak of mass distribution appeared at relatively high mass range such as equivalent to C 150. On the other hand, for a Rh/Pd doped carbon material, the observed mass distribution of clusters were limited only up to C 70. This result had a very good agreement with the fact that SWNTs from Ni/Co doped sample had larger diameter such as 1.2 nm, in contrast to about 0.8 nm from Rh/Pd doped sample. We considered that metal atoms affected the growth of the cluster, and as a result, the final SWNTs diameter distribution was determined by the size of these precursor clusters. ACKNOWLEDGEMENT This work was supported by GrantinAid for Scientific Research (Nos , , 13GS0019). REFERENCES [1] Iijima, S. and Ichihashi, T., SingleShell Carbon Nanotubes of 1nm Diameter, Nature, 363, (1993), [2] Maruyama, S., Anderson, L. R. and Smalley, R. E., Direct Injection Supersonic Cluster Beam Source for FTICR Studies of Clusters, Review of Scientific Instruments, 6112, (1990), [3] Kroto, H. W., Heath, J. R., O'Brien, S. C., Curl, R. F. and Smalley, R. E., C 60 : Buckminsterfullerene, Nature, (1985), [4] Guo, T., Jin, C., and Smalley, R.E., Doping BuckyFormation and Properties of BoronDoped Buckminsterfullerene, Journal of Physical Chemistry, 95, (1991), [5] Y. Chai, T. Guo, C. Jin, R. E. Haufler, L. P. F. Chibante, J. Fure, L. Wang, J. M. Alford and R. E. Smalley, Fullerenes With Metals Inside, Journal of Physical Chemistry, 95, (1991), [6] Kikuchi, K., Nakahara, N., Wakabayashi, T., Honda, M., Matsumiya, H., Moriwaki, T., Suzuki, S., Shiromaru, H., Saito, K., Yamauchi, K., Ikemoto, I. and Achiba, Y., Isolation and Identification of Fullerene Family C 76, C 78, C 82, C 84, C 90 and C 96, Chemical Physics Letters, 188, (1992), [7] Wilson, L. J., Cagle, D. W., Thrash, T. P., Kennel, S. J., Mirzadeh, S., Alford, J. M. and Ehrhardt, G. J., Metallofullerene Drug Design, Coordination Chemistry Reviews, 192, (1999), [8] Mikawa, M., Kato, H., Okumura, M., Narazaki, M., Kanazawa, Y., Miwa, N. and Shinohara, H., Paramagnetic watersoluble metallofullerenes having the highest relaxivity for MRI contrast agents Biocojugate Chemistry, 12 (2001), [9] Tanigaki, K., Ebbesen, T. W., Saito, S., Mizuki, J., Tsai, J. S., Kubo, Y. and Kuroshima, S., Superconductivity at 33K in CS X RB Y C 60, Nature, 352, (1991), [10] Li, Q.N., Xiu, Y., Zhang, X.D., Liu, R.L., Du, Q.Q., Sun, A.G., Chen, S.L. and Li, W.X., Biodistribution of Fullerene Derivative C60(OH)(x)(O)(y), Chinese Science Bulletin, 46, (2001), [11] Iijima, S., Helical Microtubules of Graphitic Carbon, Nature, 354, (1991), [12] Kataura, H., Kumazawa, Y., Maniwa, Y., Ohtsuka, Y., Sen, R., Suzuki, S. and Achiba, Y., Diameter Control of SingleWalled Carbon Nanotubes, Carbon, 38, (2000), [13] Yudasaka, M., Komatsu, T., Ichihashi, T., Achiba, Y., and Iijima, S., Pressure Dependence of the Structures of Carbonaceous Deposits Formed by Laser Ablation on Targets Composed of Carbon, Nickel, and Cobalt, Journal of Physical Chemistry, B, 10225, (1998), [14] Yudasaka, M., Ichihashi, T., Iijima, S., Roles of Laser Light and Heat in Formation of SingleWall Carbon Nanotubes by Pulsed Laser Ablation of C x Ni y Co y Targets at High Temperature, Journal of Physical Chemistry B, 10250, (1998), [15] Maruyama, S., Yoshida, T. and Kohno, M., FTICR Studies of Laser Desorbed Carbon Clusters, Transactions of JSME, B65639 (1999), [16] Maruyama, S., Yamaguchi, Y., Kohno, M. and Yoshida, T., Formation Process of Empty and MetalContaining Fullerene. Molecular Dynamics and FTICR Studies", Fullerene Science and Technology, 7, (1999), [17] Maruyama, S., Yoshida, T., Kohno, M. and Inoue, M., FTICR Studies of Laser Desorbed Carbon Clusters, 5th ASME/JSME Thermal Engineering Conference, San Diego, AJTE (1999). [18] Maruyama, S., Kinbara, H., Hayashi, H., and Kimura, D., PhotoIonized TOF Mass Spectrometry of Atomic Clusters, Microscale Thermophysical Engineering, 11 (1997), [19] Maruyama, S., Kohno, M., Inoue, S., FTICR Study of Chemical Reaction of Silicon Clusters, Thermal Science Engineering, 76 (1999), [20] Marshall, A. G. and Verdun, F. R., Fourier Transforms in NMR, Optical, and Mass Spectrometry, Amsterdam, Elsevier, (1990). [21] Thess, A., Lee, R., Nikolaev, P., Dai, H., Petit, P., Robert, J., Xu, C., Lee, Y.H. Kim, S.G., Rinzler, A. G., Colbert, D. T., Scuseria, G. E., Tomanek, D., Fisher, J. E., Smalley, R. E., Crystalline Ropes of Metallic Carbon Nanotubes, Science, 273, (1996), [22] Journet, C., Maser, W. K., Bernier, P., Loiseau, A., Chapelle, M. Lamy de la, Lefrant, S., Deniard, P., Lee, R., Fischer, J. E., Largescale production of singlewalled carbon nanotubes by the electricarc technique, Nature, 388, (1997), [23] Maruyama, S. and Shibuta, Y., "Molecular Dynamics in Formation Process of SWNTs", Molecular Crystals and Liquid Crystals, (2002), in print.